I don't have a cpu in mind at the moment. I'm literally building from scratch and thought the choice of motherboard would be the best place to start.
The CPU would likely be something that is comparitively energy efficient. I've read the great article at http://www.tomshardware.com/reviews/windows-7-htpc,2159-3.html
which suggested an AMD Phenom X4 9350e which is 65W so capable of running cooler and therefore quieter.